3D seismic mapping and porosity variation of intra-chalk units in the southern Danish North Sea

Abstract

Abstract Deposition of the Upper Cretaceous–Danian Chalk Group in the Salt Dome Province of the southern Danish Central Graben took place during a tectonic period dominated by post-rift subsidence, halokinesis and structural inversion. This resulted in highly variable chalk distribution with >1300 m of chalk located in synclines and <200 m preserved on inversion highs and salt structures. The area is mature with respect to exploration with most of the chalk fields located in structural traps discovered in the 1970s. However, the Halfdan discovery in 1999 illustrates the existence of off-structural traps, leading to renewed exploration interest. To locate additional off-structural traps, a detailed geological model is necessary for prediction of chalk intervals with reservoir potential. To unravel basin development, we combine 3D seismic interpretation, well log correlation and 2D seismic inversion to estimate acoustic impedance along selected profiles. The 2D acoustic impedance profiles are converted to total porosity and used to identify areas with potential untargeted reservoirs. A prominent high-amplitude reflection is interpreted as a regional unconformity separating two distinctly different chalk deposition patterns. Nannofossil biostratigraphy suggests a latest Campanian to early Maastrichtian age for the unconformity. It corresponds to an increase in acoustic impedance and decrease in porosity in wells. The Tor Formation contains porous intervals while the underlying Hod Formation contains less porous chalk. The Hod Formation has a maximum porosity of <20% based on well log and inversion data. In contrast, inversion data indicate that the Tor Formation comprises reservoir-grade porosity at several locations on downflank structures. In several areas, the inversion-based maximum porosity is predicted to be higher than expected, compared with porosity/depth trends derived from well data. Therefore, the spatial porosity variation in chalk is complex and controlled by factors other than burial depth.